Historically, circuit protective devices, such as circuit breakers, have used electromechanical elements responding to an overcurrent condition in a protected circuit pursuant to initiating automatic tripping of the circuit breaker and interruption of the circuit. When the overcurrent condition is of heavy overload or short circuit proportions, it is necessary to trip the circuit breaker virtually instantaneously, and to this end, electromagnetic trip initiating elements have traditionally been utilized. However, for less severe overcurrent conditions, it is desirable to initiate tripping of the circuit breaker after a time delay which is inversely related to the magnitude of the overcurrent. The trip initiating element most widely used to affect such delayed tripping is a bimetal, although electromagnetic tripping mechanisms equipped with suitable mechanical response retardants means, such as dashpots, are commonly used in the larger industrial type circuit breaker. The bimetal is especially suited to this end, since it can be structured such that its mechanical response time is inversely proportional to the second power of the circuit current. This operating characteristic, where the square of the circuit current multiplied by time equals a constant (I.sup.2 t = K), is ideal since it conforms to the thermal damage characteristic of the protected circuit.
In recent years, the requisite trip-time curve operating characteristic for circuit breakers has been achieved using electronic or "static" trip units. These static trip units utilize combinations of semi-conductor elements and other physically small, low-power solid-state components having no moving parts. The attractiveness of static trip units resides, in part, in low power consumption, environmental stability, and the convenient adjustability of trip settings.
Current static trip unit designs almost invariably utilize an analog circuit approach, wherein energy storage means in the form of a reactive element, such as a capacitor, is implemented to provide the requisite delay which varies inversely as the square of the circuit current. It is a principle object of the present invention to provide a static trip unit which utilizes a predominantly digital circuit approach to the same end. Other objects of the invention will in part be obvious and in part appear hereinafter.